1. Field of the Invention
The present invention relates to a method of evaluating a golf club, and to a golf club which has been evaluated in accordance with the method of evaluating a golf club. In particular, the present invention relates to a method of evaluating a golf club, and a golf club, in which the size of a sweet area of the golf club is able to be accurately indicated, and to a golf club set having a plurality of such golf clubs in which the golf clubs differ in loft angle and in club length.
2. Description of the Related Art
Golfers often rely upon the golf clubs in order to obtain a good score, in addition to improving their skill in order to achieve good golf play. A good performance of the golf clubs is therefore always desired by many golfers. The good performance, for example, has a meaning of long distance of a golf ball flight and ease of hitting a golf ball. In particular, the ease of hitting is evaluated from a subjective standpoint along with an objective standpoint in which the variation of the golf ball flight becomes small between cases of hitting a golf ball at the center of the golf club face and cases of hitting a golf ball off the center. This is generally discussed by the size of the sweet area. Namely, golf clubs are evaluated such that the larger the sweet area of the golf club, the smaller the variation (drop in flight distance, and lateral shift of a golf ball flight) becomes between cases of hitting at the center and cases of hitting off the center.
The size of the sweet area of the golf club depends upon the golf club head, and as stated later, is mainly evaluated in accordance with the numerical value of a moment of inertia about a specific axis of the golf club head itself. In other words, from the numerical value of the moment of inertia, the degree of a steady performance of the golf club head when the golf club head hits a golf ball off a sweet spot can be obtained, and the larger the numerical value of the moment of inertia, the less unsteady performance the golf club head has in hitting off the center, and the more similar the golf ball flight of hitting off the center comes to a golf ball flight of hitting at a sweet area. Namely, when the difference between cases of hits at the center of the golf club and cases of hits off the center becomes small, it can be evaluated that the golf club has a large sweet area.
In Japanese Patent No. 2851542, a technique is disclosed for increasing the moment of inertia with respect to a golf club head, in a state in which its sole is placed in a horizontal plane, about a horizontal axis parallel to the plane of the club face through the center of mass (moment of inertia about an X axis in the above patent), and the moment of inertia about a vertical axis through the center of mass (moment of inertia about a Y axis in the above patent) , thereby enlarging the sweet area, by setting the thickness and the volume of the golf club head.
In particular, the most generally used value of the moment of inertia by those skilled in the art is a value of the moment of inertia about the vertical axis parallel to the vertical direction through the center of mass of the golf club head, also shown in the above Japanese Patent No. 2851542. The value of the moment of inertia measured around this axis is generally used for evaluating the size of the sweet area.
The above axes about which the moments are measured are selected on the assumption that the golf club head rotates in directions when the sweet area of the golf club head is missed. For example, the axis parallel to the plane of the hitting surface of the golf club head, through the center of mass, and in a horizontal direction shown in Japanese Patent No. 2851542 (the X axis shown in FIG. 1 of the above patent) is an axis of rotation of the golf club head when a golf ball is hit while missing the sweet area up and down in a vertical direction. The axis in a vertical direction through the center of mass of the golf club head (the Y axis shown in FIG. 1 of the above patent) is an axis of rotation of the golf club head when a golf ball is hit while missing the sweet area of a golf club head in a toe and heel (lateral) direction.
When an ordinary golfer misses the sweet area while hitting there is no certain tendency of the miss, in the toe and heal direction or in the vertical direction, and generally hitting points on the golf club surface are variously varied around the sweet area.
However, the moment of inertia about the axis parallel to the plane of the hitting surface of the golf club head, through the center of mass, and in a horizontal direction, such as that of the above Japanese Patent No. 2851542, can only indicate the steady performance of the golf club head when hitting while missing the sweet area in a vertical direction, namely the size of the sweet area in the vertical direction. Further, the moment of inertia around the axis in a vertical direction through the center of mass can only indicate the steady performance of the golf club head when hitting while missing the sweet area in a toe and heel direction, namely the size of the sweet area in the toe and heel direction.
The two moments of inertia of the above Japanese Patent No. 2851542 therefore cannot be used as indicators for unambiguously and accurately evaluating the size of the sweet area based upon a golfer""s tendency that the hitting points are variously varied around the sweet area.
In other words, although the golf club head can be made to have relatively steady performance even in cases in which an ordinary golfer hits while missing the sweet area, and a golf ball flight relatively similar to a golf ball flight that occurs when hitting the ball at the sweet area can be stably realized, by evenly increasing the moment of inertia about the axis around the toe and heel direction, and the moment of inertia about the axis of the up and down direction, it is difficult to accurately increase the size of the sweet area quantitatively by increasing these moments of inertia, and it is difficult to accurately indicate the size of the sweet area by these moments of inertia.
Additionally, golfers have different skill levels such as those of beginners, intermediate golfers, and further advanced golfers. For example, beginners who often hit a golf ball while missing the sweet area, wants to select a golf club which will make almost the same golf ball flight as that of hitting at the sweet area, even when the sweet area is missed, and which impresses beginners a good sense of stability. On the other hand, there are many times when an advanced golfer will intentionally hit the golf ball off the sweet area in order to control the flight of the golf ball by subtly changing the striking direction of the ball and imparting spin to the ball. Advanced golfers desire to select a golf club with good controllability in which the ball flight can be controlled by the extent that a golf ball is hitted off the sweet area intentionally.
Although there are golf clubs classified toward advanced golfers, intermediate golfers, and beginning golfers in accordance with their level of skill, the golfer must evaluate a golf club subjectively by hitting it numerous times in order to select a golf club having a suitable size of the sweet area for that golfer. However, the evaluation cannot be obtained only from the size of the golf club sweet area, but from factors such as the moment of inertia around the axis of the golf club shaft, and the set value of the lie angle of the golf club head and the like, as the overall performance.
Therefore, whether a golf club is one impressing golfers sense of stability and producing results almost the same as those of hitting at the sweet area even when the sweet area is missed, whether it is one impressing golfers good controllability in which the ball flight can be controlled by the extent at which the sweet area is missed, or whether the golf club is one located between these characteristics, discerning the stability and controllability of golf clubs and selecting a golf club which is suitable for golfers is difficult for golfers.
On the other hand, golfers play golf using a plurality of such golf clubs, at most fourteen golf clubs having differing club lengths and differing loft angles.
For example, wood type golf clubs such as a number 1, number 3, and number 5 wood, numbers 3 through 9 irons, a pitching wedge and a sand wedge are arranged, and the golf clubs are suitably selected and hit a golf ball based upon the distance to the pin.
In particular, in order to make accurate shots of the golf ball to the pin, it is preferable to control the flight distance accurately in accordance with iron club number.
However, the ball flight distance changes as stated above for cases in which the club head is hit at the center and for cases in which the club head is not hit at the center, and therefore variation in the ball flight distance appears. Many types of golf clubs having a wide sweet spot and impressing a sense of stability, and with which the shot results are almost similar to those of a sweet area hit are obtained even when the center is missed, have therefore been proposed.
However, there are many golfers who, from a strategy for playing a golf in a golf course, prefer to utilize the lateral shift of a ball flight when they strike the golf club for small loft angles of golf clubs, thereby controlling the ball flight, while relatively suppressing the variation in ball flight distance and variation in the lateral shift of the struck ball when the golf club is hit off the center, and at the same time prefer to suppress the lateral shift of a ball struck by a golf club having a large loft angle. Further, there is also a lot of demand even among such as beginner golfers, who easily hit off the center, for golf clubs with which more accurate shot can be made toward the pin as the golf club number increase. However, at present it is not well understood how to adjust the sweet areas of a plurality of golf clubs, such as irons in accordance with the club number, with respect to the demands of those types of golfers.
Further, when a beginner golfer hits who easily misses the center, or an intermediate to advanced golfer hits who may miss the center, using a golf club set in which the sweet area of a 6 iron, for example, has a sweet area which is large compared to a 5 iron, a result tends to occur that the 6 iron performs a relatively long flight distance in comparison to a flight distance which is set in a manner that the flight distance with respect to golf club number changes by approximately 10 yards for every change by the club number. In addition, the difference in ball flight distance between the 6 iron and a 7 iron becomes greater. There are therefore many cases in which it is extremely difficult to control the flight distance based upon the number of the golf club. In addition, although in general the demand for an accurate shot directed toward the pin increases with increasing the golf club number, a problem develops in that, compared to the number 6 iron, an accurate shot cannot be make with the number 7 iron. This type of golf club set, and these types of golf clubs, therefore have incompatibilities in that they do not sufficiently perform desirable function corresponding to their club number, and in addition impart distrust to the golfer.
An object of the present invention is therefore to provide a method of evaluating a golf club in which it is possible to accurately indicate the size of a sweet area of the golf club, which influences the golfer""s impression of stability and controllability of the golf club, and a golf club given an evaluation in accordance with the method of evaluation. In addition, an object of the present invention is to provide a golf club set composed of a plurality of golf clubs having differing loft angles, in which variation in ball flight distance is regulated with respect to the number of the golf club based upon an index accurately indicating the size of a sweet area.
To solve the problems, the present invention provides a method of evaluating a golf club having a golf club head, comprising a step of measuring a moment of inertia about an axis perpendicular to a face plane of the golf club head through a center of gravity of the golf club head and a step of evaluating the golf club by the measured moment of inertia.
In the invention, the golf club may be evaluated by dividing the measured moment of inertia by a mass value of the golf club head.
The present invention also provides a golf club comprising a golf club shaft having a golf club head at one end and a grip or a grip portion at an opposite end to the end of the golf club head, wherein the golf club has an evaluation imparted based upon a moment of inertia about an axis perpendicular to a face plane of the golf club head through a center of gravity of the golf club head.
It is preferable that moment of inertia information relating to a value of the moment of inertia; or moment of inertia information relating to a value which is obtained by dividing the value of the moment of inertia by a mass value of the golf club head is indicated on a portion of the golf club.
In the invention, the moment of inertia information is used in order to obtain a golf club evaluation information by referring to a reference information in which the moment of inertia information and the golf club evaluation information correspond to each other.
Preferably, the reference information for obtaining the golf club evaluation information, based upon the moment of inertia information, is displayed on a portion of the golf club along with the moment of inertia information.
The golf club may be classified into a type based upon a value of the moment of inertia, or upon a value obtained by dividing the value of the moment of inertia by a mass value of the golf club head.
Further, the invention provides a golf club set comprising at least three golf clubs having different loft angles, wherein each of the golf clubs has the size of the moment of inertia about an axis perpendicular to a face plane of a golf club head through a center of gravity of the golf club head adjusted corresponding to an order of golf club number, or corresponding to an order of value of loft angle.
It is preferable that the moment of inertia changes almost linearly in correspondence with the order of the golf club number.
It is also preferable that the golf club set has a group of at least three golf clubs having loft angles in a range greater than or equal to 16xc2x0 and less than or equal to 41xc2x0, and when each of the golf clubs of the group is expressed as continuous natural number X, beginning with a smallest golf club number which is taken as X=1, and when the moment of inertia is taken as Y (gxc2x7cm2), then the moment of inertia Y of each of the golf clubs of the group is adjusted within a range defined by equation (1) below, with respect to the natural number X:
axc2x7X+bxe2x89xa6Yxe2x89xa6axc2x7X+b+50xe2x80x83xe2x80x83(1)
where coefficients a and b are constants.
Then, the coefficient a of the equation (1) may be equal to or less than 60, for a golf club set suitable to some golfers.
Alternatively, the coefficient a of equation (1) maybe greater than 60, for a golf club suitable to other golfers.
It is preferable that the golf club set has a group of at least three golf clubs having loft angles in a range greater than or equal to 16xc2x0 and less than or equal to 41xc2x0, and when each of the golf clubs of the group is expressed as continuous natural numbers X, beginning with a smallest golf club number which is taken as X=1, and when a distribution of the moments of inertias of all of the golf clubs of the group with respect to the natural number X is regressed by a regression line, the sizes of the moments of inertia of all of the golf clubs of the group are adjusted such that all of estimated errors in the regression line of the moments of inertias of all of the golf clubs of the group are equal to or less than 30 (gxc2x7cm2).
It is preferable that the moment of inertia changes almost linearly in correspondence with the size of the loft angle of each of the golf clubs.
It is also preferable that the golf club set has a group of at least three golf clubs having loft angles in a range greater than or equal to 16xc2x0 and less than or equal to 41xc2x0, and when the loft angle of each of the golf clubs of the group is taken as xcex8xc2x0, and the moment of inertia is taken as Y (gxc2x7cm2) , then the moment of inertia Y of each of the golf clubs of the group is adjusted within a range defined by equation (2) below, with respect to the loft angle xcex8:
xe2x80x83cxc2x7xcex8+dxe2x89xa6Yxe2x89xa6cxc2x7xcex8+d+50,xe2x80x83xe2x80x83(2)
where coefficients c and d are constants.
Then, the coefficient c of the equation (2) may be equal to or less than 15 for a golf club set suitable to some golfers.
Alternatively, the coefficient c of the equation (2) may be greater than 15 for a golf club set suitable to some golfers.
It is preferable that the golf club set has a group of at least three golf clubs having loft angles in a range greater than or equal to 16xc2x0 and less than or equal to 41xc2x0, and when a distribution of the moments of inertias of all of the golf clubs of the group with respect to the loft angles is regressed by a regression line, the sizes of the moments of inertias of all of the golf clubs of the group are adjusted such that all of estimated errors in the regression line of the moments of inertias of all of the golf clubs of the group are equal to or less than 30 (gxc2x7cm2).